Wheeled vehicles often utilize a suspension system, at one or more corners of the vehicle, in which a wheel is attached to a chassis of the vehicle by several suspension elements, including a generally horizontally extending articulating control arm. The control arm has an inboard end thereof attached in an articulating manner to the chassis, and an outboard end thereof attached in an articulating manner to a steering knuckle, or other structure, including an axle supporting the wheel. Having the inboard and outboard ends of the control arm articulated in this manner allows the inboard and outboard ends of the control arm to move up and down with respect to one another, during operation of the vehicle.
One or more compressible suspension elements, such as a spring or a damper, are also typically connected between the chassis of the vehicle and a fixed intermediate attachment point on the control arm, at a point located between the inboard and outboard ends of the control arm, for supporting the vehicle on the suspension and limiting motion of the suspension during various operating conditions of the vehicle.
In the majority of suspension systems used in the past, the various elements making up the suspension system are passive, in that once they are manufactured and installed in the suspension system, their performance parameters are not adjusted, during operation of the vehicle, to actively tune the suspension to accommodate variables such as differing road surfaces, operating speeds, or how heavily or lightly the vehicle may be loaded. The ability to actively tune a suspension is highly desirable.
Some prior suspension systems have used various approaches to provide active suspension tuning, but these approaches have generally been too complex and costly to find wide acceptance and use. In one prior approach, compressible suspension elements include springs or dampers having cavities for containing compressed air, supplied by an onboard compressor and pneumatic system. In a similar prior approach, compressible suspension elements include springs or dampers having cavities for containing variable amounts of hydraulic fluid, supplied by an onboard hydraulic pump and hydraulic system. An onboard computer controls the compressible suspension elements in response to signals generated by sophisticated system of sensors placed throughout the vehicle. By varying the amount or the pressure of air or hydraulic fluid in the cavities of the compressible suspension elements, the height of the chassis can be varied, to compensate for changes in vehicle loading, or the stiffness of the suspension at the corners of the vehicle can be varied to provide improved handling during maneuvering of the vehicle. The need to carry a hydraulic or pneumatic system, and the bulkiness and cost of the compressible suspension elements have limited the use of such prior actively tuned suspension systems.
In another more recent approach to providing an actively tunable suspension system, hydraulic dampers in the suspension systems are filled with a special magnetorheological (MR) fluid, which changes viscosity when exposed to a magnetic field, and the dampers are equipped with electrical coils for impressing a magnetic flux on the MR fluid. By varying the electrical current provided to the coils, the viscosity of the MR fluid can be actively controlled, which in turn causes the damper to become more, or less stiff, depending upon the level of electrical current that is applied. While such MR damping systems work well, and are considerably less complex and smaller in size than other prior approaches using hydraulic or pneumatic systems, the cost of the MR fluid and MR dampers is high enough that their use has been limited to only a few relatively high priced vehicles.
What is needed is an improved method and apparatus for providing an actively tunable vehicle suspension, that overcomes one or more of the problems described above.